Transmission electron microscope sample preparation

ABSTRACT

Sample preparation apparatus and method includes a wafer stage platform with an optical microscope and integrated pattern recognition to automatically address specific locations on the wafer sample of interest. A laser attaches to the optical microscope to mill a set pattern around the area of interest. A precision micro-manipulator engages the sample support structure, extracts the structure, and places the structure in a TEM holder or holder tip. The holder or holder tip can then be placed inside a FIB for final thinning, followed by direct transfer into the TEM.

This application claims the benefit of U.S. Provisional Application No.60/222,728 filed Aug. 3, 2000.

BACKGROUND OF THE INVENTION

Transmission electron microscopes (TEMs) have been used in a variety ofscientific disciplines for 50 years. A TEM works in a way fundamentallysimilar to a light microscope except for the use of an electron beaminstead of light. Electrons have a much shorter wavelength andconsequently allow viewers to see features approaching atomic size (<1nanometer) in comparison to a limit of 100 nm (0.1 μm for light).

Over the last 50 years, transmission electron microscopy (TEM)technology and its applications have undergone tremendous advances. TEMhas become an important analytical tool in many disciplines that requirevisualization and/or imaging of features less than one nanometer insize, a resolution at least two orders of magnitude greater than thatavailable to optical microscopy.

Unfortunately, the preparation of samples for TEM analysis is fraughtwith difficulties. TEM only works with samples made thin enough to betransparent to electrons, a distance on the order of 50 nm. Thinningsamples through cutting or grinding is especially difficult for hard,tough, and especially brittle materials; furthermore, attempts to meeteven reasonable standards of reproducibility and throughput often fail.The bottom line is that successful TEM sample preparation generallyrequires the use of highly trained, reliable, experienced, and, hencevery expensive technical personnel.

There are many practical difficulties inherent to using a transmissionelectron microscope (TEM) to image microstructures. Samples must firstbe thinned to an ultra-thin membrane transparent to electrons:approximately 50 nm (0.00005 mm) or less. This is less than {fraction(1/1000)} of the diameter of a human hair. In reality, this is extremelydifficult to accomplish on hard materials since many crystallinematerials, such as silicon, do not lend themselves to conventionalcutting or grinding techniques. Additionally, one cannot easily observethe area of interest while thinning it, and handling the minusculesamples requires a highly trained, dexterous technician.

In the past ten years, the market for TEMs has been static at about 250units per year (100 in the U.S.). As Government funding decreased forlife sciences in general, demand for biological TEMs declined.Offsetting this trend, high-end analytical TEMs for materials. (metals,ceramics, semiconductors, superconductors, etc.) increased, largely dueto the need to view crystalline structures and thin film interfaces atthe highest possible visual acuity. The demand for TEM images continuesto grow precipitously in the semiconductor market segment as companiesmove to smaller design rules and deeper vertical integration.

The increasing demand for TEM does not necessarily translate directlyinto instrument sales, since the real bottleneck in productivitycontinues to be the tedious and time-consuming nature of samplepreparation. Several labs have commented that their TEMs sit idle muchof the time, waiting for quality samples of the right area to beprepared.

There are significant differences in requirements between customers whoextract samples from full wafers vs. customers who analyze samples frombulk materials. The bulk sample customers represent a wide cross-sectionof analytical electron microscope users in a broad range of materialsapplications in institutional and industrial settings. Although in themajority, they do not have the same critical need for productivity thatsemiconductor customers have. The wafer customers, best represented byprocess development and yield engineers, have the following generalrequirements: 1) they want cross sections of specific devices or defectsextracted directly from wafers; 2) they want specific locationcoordinates transferred from other equipment; 3) they want samplepreparation, from full wafer to imaging, to be as fast as possible; 4)they want confidence that an exact location on a wafer can be sampled;5) they want a minimum of artifact formation; 6) they want the processautomated to the fullest possible extent; and 7) they do not want tosacrifice the entire wafer for a single sample preparation.

Within the current general approach of TEM sample preparation, there aresubtle variations; however, each consists of a technique for removing abulk sample from the wafer, reducing the area of interest to anultra-thin membrane, and finally transferring the membrane to the TEM.The current approaches to TEM sample preparation include using highprecision diamond saws, broad beam thinning techniques and single beamfocused ion bean (FIB) milling methods. In most these approaches, thesample must be manually transferred to the TEM for analysis.

High precision diamond saws are currently available for TEM samplepreparation. Each claims to be able to reach the right spot on a waferautomatically and create an approximate 1 μm×1 μm block containing thesample. The process takes anywhere from 30 minutes to two hours,depending upon the skill of the technician. Clearly, the major drawbackto this type of apparatus is the fact that the wafer is sacrificed, andonly one sample per wafer is utilized.

The broad beam thinning technique of TEM sample preparation is virtuallyobsolete for full wafer sample extraction simply because it is slow, thearea of interest cannot be targeted, and the ultimate thickness isvariable.

There are two different single beam FIB approaches for TEM samplepreparation. In one approach, multiple samples can be automaticallyprocessed up to, but not including the final cut. After thinning,samples must then be manually transferred to a support grid and into aTEM holder. Some commercial TEM sample preparation methods offer adetachable tip “FIB-EM” that can be pre-mounted on the FIB stage beforethinning in an attempt to reduce transfer damage.

A second approach for single beam FIB sample preparation allows users toinsert a TEM sample rod into an FIB through an airlock, thus avoidingmanual handling of the delicate thin section any time after it ismilled. This reduces the chance of physical damage.

The use of a dual beam (combination FIB and SEM) for sample thinning andextraction has become the dominant technique or wafer applications. Thearea of interest of the wafer can be located using navigation software,and ultra-thin sections can then be cut directly from the wafer. Thistechnique completely avoids the intermediary diamond saw and lappingsteps. Unfortunately, the ultra-thin membrane itself must be lifted fromthe wafer and transferred to a grid manually in this approach.

There are three methods available for transferring the membrane fromwafer to TEM sample holder. All three permit the user to preparemultiple samples from one wafer without destroying the rest of thedevices on the wafer. One approach allows for picking up thin sectionsof TEM samples by electrostatic attraction. In this method, FIB preparedsamples are manually located under a binocular microscope. The membraneis then touched with a charged micro-manipulator probe in the hope thatstatic attraction between the thin membrane and the probe will occur.Even with an expert technician at the controls, membranes are likely todisappear or crumble during the manual transfer.

A second technique involves grabbing the membrane inside the dual beamwith a micro-tweezer. If the membrane survives the detachment from thesubstrate and tweezer jaws, the operator must then gingerly place themembrane on a TEM grid. The TEM grid must then be transferred to the TEMsample holder. Although there is better visual observation possible atthe extraction site, there is also significant increased physicalhandling.

The newest technique was developed by Tom Moore, and is called the MooreTechnique. In that approach, a probe is actually welded to the finishedmembrane inside the FIB using the FIB's metal deposition capability. TheFIB then cuts the membrane free from the matrix. It can then betransferred to a grid, where the probe weld is cut, and the membrane canthen be welded to the grid.

Needs exist for a TEM sample preparation that is simple, cost effective,and automated to decrease the risk of human error when transferringsamples to a TEM for analysis .

SUMMARY OF THE INVENTION

In earlier years, TEM sample preparation often required more than a day.Recently, the application (principally by the semiconductor industry) offocused ion beam (FIB) milling technology to TEM sample preparation hasshortened that time to about three hours. With the approach of thepresent invention, TEM sample preparation time is cut from about 30minutes to about two hours, or between a 16% to 67% reduction inpreparation time.

Currently, TEM sample preparation for semiconductor devices is typicallydone with diamond saws, microtweezers, and finished with a focused ionbeam (FIB) milling.

This invention replaces the diamond saw and cut or machine with a laser.Typical laser machining leaves thermal damage on the micrometer scale.Since the device size scale is micrometers and less, thermal damage isunacceptable. To eliminate thermal damage caused by laser ablation, afemtosecond (one quadrillionth or 10⁻¹⁵ second) laser is used in thepresent invention.

Femto-laser machining is not well known; however, it has been done onvarious materials for a couple of years. The present invention is a toolwith femto-laser machining capability. The laser cuts a sample out of awafer that is strategically placed to handle the cut piece. The cutpiece is on the order of 5 μm by 10 μm by the thickness of the wafer,750 μm. The piece must be turned on its edge and trimmed back. The finalpolishing or thinning may be done with an FIB milling. The laser cuttingor milling leaves a thin layer between two large ends at the top of alarge supporting blade of the test material. The block is turned on itsside so that the thin layer may be examined by a TEM.

The advantage of the laser milling technology of the present inventionover the present state of the art is that it does not require theremoval of a fragile membrane from a silicon wafer, which is the casewith current FIB TEM sample preparation. Rather, a laser removes ashaped block from the wafer which contains an area thinned for TEM andguide holes for mounting to a bracket designed to fit both FIB and TEMinstruments.

The laser-milling instrument of the present invention consists of awafer stage platform connected to an optical microscope and acomputer-operated integrated-pattern-recognition assembly, a millinglaser, and a precision micromanipulator.

The laser-milling instrument of the present invention as a wholerepresents an innovation, with the newest portion including the laseritself. The phenomenon of laser ablation of silicon has been studied foryears, but the results with older nanosecond-regime (NR) lasertechnologies would not suffice for the small structures of current andnear-future silicon integrated-circuit technologies. Lasers operating inthe sub-picosecond regime (SPR) are relatively new inventions, and haveonly been applied to the ablation of silicon since about 1995. Theselasers offer the promise of superior performance, and ease ofapplication to milling.

The present invention is a new technique having a number of advantagesover the current commercially available solutions. Instead of trying toextract a fragile, minuscule membrane from a wafer after FIB processing,a laser is used to cut out a uniform sample shaped block containing thethinned area of interest and a pair of laser drilled holes. The holesare mated to twin probes mounted on a bracket that can be fitteddirectly to a FIB stage or modified TEM holder. The membrane isprotected by the bracket throughout the preparation and transfer stepsof the process. Because the sample has been pre-cut with a precisionlaser, there is a minimum of actual trimming required in the FIB,eliminating 45 minutes to two hours of milling time.

The essential difference between this and all other currently availabletechniques is that the surrounding silicon substrate of the wafer isused as part of the support strategy for protecting the FIB preparedthin membrane.

A preferred embodiment has a wafer stage platform with an opticalmicroscope and integrated pattern recognition to automatically addressspecific locations on the wafer. Preferably, a laser attaches to theoptical microscope to mill a set pattern around the area of interest.Finally, there is a precision micro-manipulator that engages the samplesupport structure, extracts the structure, and places the structure in aTEM holder or holder tip.

The process of the present invention provides a safe, easy to use,semi-automated, and efficient method for cutting and extracting aspecific nano-level feature from a wafer. The method allows a user toremove most of the material in situ, while also providing protection forsubsequent steps. The holder or holder tip can then be placed inside aFIB for final thinning, followed by direct transfer into the TEM.

The TEM sample preparation of the present invention results in lowercosts per sample. A comparison of the costs involved with TEM samplepreparation using the present invention versus using other currentlyavailable methods is summarized in Table 1.

These and further and other objects and features of the invention areapparent in the disclosure, which includes the above and ongoing writtenspecification, with the claims and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a preferred embodiment of the TEM samplepreparation apparatus of the present invention.

FIG. 2 is a perspective view of a laser beam cutting and drilling anarea of interest from a wafer.

FIG. 3 is a perspective view of a bracket with dual stylus lifting acarrier block from a wafer.

FIG. 4 is a perspective view of a cut wafer on a TEM Holder formanipulation by an FIB.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the present invention provides a safe, easy to use,semi-automated, and efficient method for cutting and extracting aspecific nano-level feature from a wafer. The method allows a user toremove most of the material in situ, while also providing protection forsubsequent steps. The holder or holder tip can then be placed inside aFIB for final thinning, followed by direct transfer into the TEM.

As shown in FIG. 1, a preferred embodiment of the TEM sample preparationapparatus of the present invention has a wafer stage platform 14 and anoptical microscope 16. A computer operated pattern recognition assembly18 is connected to the optical microscope for automatically addressingspecific locations of areas of interest 4 on the wafer 1 as selected bythe optical microscope. Preferably, a milling laser 20 is attached tothe optical microscope 16 to mill a set pattern around an area ofinterest 4 of the wafer 1. The pattern includes forming a thin samplestrip 15 with ends 5 and holes 6, followed by cutting the block 13 fromthe wafer.

The phenomenon of laser ablation of silicon has been studied for years,but the results with older nanosecond-regime (NR) laser technologiesoperating at one billionth of 10⁻⁹ seconds would not have sufficed forthe small structures of current and near-future siliconintegrated-circuit technologies. Lasers operating in the sub-picosecondregime (SPR) are relatively new inventions, and have only been appliedto the ablation of silicon since about 1995. These lasers offer thepromise of superior performance, and ease of application to milling.

Table 1 shows an overview and comparison of TEM sample preparationprocedures including that of the present invention.

The milling laser 20 is preferably a femto-laser. Typical lasermachining leaves thermal damage on the micrometer scale. Since thedevice size scale is micrometers and less, thernal damage isunacceptable. To eliminate thermal damage caused by laser ablation, afemtosecond laser 20 is used in the present invention. Using afemto-laser as the milling laser 20 minimizes thermal damage caused bylaser ablation.

A micro-manipulator 21 moves the milling laser 20 to form the samplestrip 15 and cut the block 13 from the wafer 1. Alternatively, amicro-manipulator 19 moves the wafer platform during laser milling andcutting.

A precision micro-manipulator 22 moves an arm 8 with dual stylus 9 toengage the sample support structure ends for extracting the cut block 13from the wafer 1 and places block 13 in a TEM holder tip 26 of a TEMholder 10. The sample strip is then finished and thinned by an FIB 12.The cut wafer block 13 is then rotated 90° by the TEM holder 10 and istransferred to a TEM 28 for analysis.

As shown in FIG. 2, 3, and 4, the area of interest 4 is trimmed to apre-set pattern 11 using a laser optimized for cutting silicon from thewafer 1 creating thin sample 15. A bracket 8 with twin stylus 9 engagesand lifts the carrier block 13 from the wafer 1. The carrier block 13with sample 15 attached are transferred on holder 10 for FIB 12 thinningand TEM inspection without touching sample 15. Holder 10 is then rotatedfor TEM viewing of the sample 15.

To prepare a TEM sample 15, the milling laser 20 cuts the sample out ofa wafer 1 using a laser beam 2, as shown in FIG. 2. The wafer 1 isstrategically placed to withstand the cutting. The cut block 13preferably is approximately 5 μm by 10 μm the thickness of the wafer 1,which is 750 μm. The cut area of interest 4 must be turned on its edgefor inspection. It may be trimmed first and subjected to final polishingor thinning with an FIB 12.

The advantage of the laser milling technology of the present inventionis that it does not require the removal of a fragile membrane from asilicon wafer 1, which is the case with current FIB TEM samplepreparation. Rather, the cutting and milling laser 2 removes a shapedblock 13 from the wafer 1.

The shaped block contains a sample strip 15 in a selected area 4 thinnedfor TEM inspection. Ends 5 with laser-drilled guide holes 6 for pickingup by an arm or bracket 8 with dual stylus 9, as shown in FIG. 3, isdesigned to fit both FIB 12 and TEM 28 instruments. In this way, the TEMsample preparation of the current invention uses the silicon substrateof the wafer block 13 to protect the thin membrane sample 15 taken fromthe area of interest 4 in the wafer 1.

As shown in FIG. 4, once a wafer 1 has been cut and lifted by thebracket 8 with dual stylus 9, which is controlled by micro-manipulator22. The cut wafer block 13 is transferred to a TEM holder 10 or a TEMholder tip 26. The cut wafer block 13 is then subject to manipulation byan FIB 12 to prepare the sample for analysis by a TEM 28.

With the present invention, TEM sample preparation time is cut fromabout 45 minutes to about two hours, up to a 67% reduction inpreparation time.

While the invention has been described with reference to specificembodiments, modifications and variations of the invention may beconstructed without departing from the scope of the invention, which isdefined in the following claims.

1. A sample preparation method comprising providing a sample wafer ofinterest on a support, cutting and extracting a desired nano-levelportion from the wafer with a laser, holding and protecting the portionwith a holder, placing the portion inside a focused ion beam source,thinning an area of interest on the portion with the focused ion beamsource, and transferring the portion with the area of interest to atransmission electron microscope and analyzing the portion with thetransmission electron microscope.
 2. The method of claim 1, wherein theextracting comprises extracting the portion in situ.
 3. The method ofclaim 1, wherein the providing the sample comprises providing a waferstage platform for the sample, and selecting the area of interest withan optical microscope.
 4. The method of claim 3, further comprisingautomatically addressing specific locations of the area of interest onthe wafer selected by the optical microscope with a computer operatedpattern recognition assembly connected to the optical microscope.
 5. Themethod of claim 4, further comprising milling the wafer with the laserattached to the optical microscope, wherein the milling comprisesmilling a desired pattern of the area of interest in the wafer.
 6. Themethod of claim 5, wherein the milling comprises milling with afemto-laser for minimizing thermal damage caused by laser ablation. 7.The method of claim 5, wherein milling the desired pattern comprisesforming a thin sample strip with ends and holes and then cutting theportion as a block from the wafer.
 8. The method of claim 7, furthercomprising moving the sample wafer with a micro-manipulator during thelaser milling and the cutting.
 9. The method of claim 8, wherein themoving comprises moving the laser during the laser milling and thecutting.
 10. The method of claim 8, wherein the moving comprises movingthe support during the laser milling and the cutting.
 11. The method ofclaim 8, wherein the moving comprises moving an arm with dual stylus ofthe micro-manipulator, engaging ends of the milled portion, extractingthe cut block from the wafer, placing the block in a transmissionelectron microscope holder tip of a transmission electron microscopeholder.
 12. The method of claim 11, further comprising finishing andthinning the sample strip by a focused ion beam source.
 13. The methodof claim 12, further comprising rotating the block by about 90° with thetransmission electron microscope holder and transferring the block to atransmission electron microscope for analysis.
 14. The method of claim13, further comprising turning the area of interest on its edge,inspecting and analyzing the area of interest with the transmissionelectron microscope.
 15. The method of claim 7, wherein the milling theblock comprises forming a block of desired shape containing the samplestrip in a selected area, forming the sample strip thinner than theblock, and inspecting the sample strip with a transmission electronmicroscope.
 16. The method of claim 15, further comprisinglaser-drilling guide holes on ends of the block for picking up by an armor bracket with dual stylus and using a substrate of the block forprotecting the thin sample strip taken from the area of interest of thewafer.
 17. The method of claim 16, further comprising fitting focusedion beam source and transmission electron microscope instruments on thebracket for automatically receiving the block without any contamination.